Because you are creating a web app first, you can generally test your apps in any main browser. If it helps you can resize the browser window to the various dimensions of target devices to ensure your app still looks good. Then, once you drop the HTML5 web app into Eclipse, you can test it in the emulator, and on a local Android device if you have one, before finally uploading the finished app.

And the same goes Xcode on iOS too, using the exact same web app, so you don't have to write the app twice.